Brokerage Giant Needs Little Guys

By

Jenny Strasburg And

Jacob Bunge

Updated Dec. 19, 2012 11:04 a.m. ET

Knight Capital Group's takeover by rival Getco LLC, confirmed on Wednesday, creates a brokerage powerhouse with control over a large chunk of all U.S. stock trades. But it also raises a question voiced by one of Knight's biggest customers during the deal talks: Will the combination be good for small investors?

The issue was raised by Fred Tomczyk, chief executive of retail brokerage TD Ameritrade Holding Corp. and a Knight director,...